With Kepler-type erect image viewfinders using a porro-prism, it is known to enhance image magnification by splitting the porro-prism into two prisms and providing the image-forming plane of the objective lens between the two prisms. With this arrangement, an eyepiece having a short focal length can be used to view the image on the image-forming plane, whereby the image magnification can be enhanced. However, the viewfinders using this split porro-prism arrangement have a problem in that compensating for field curvature is difficult. A field flattener used to compensate for field curvature works effectively if the flattener is positioned near the image-forming plane. However, this positioning of the flattener is impossible if the image-forming plane is provided between the first and second prisms.
To solve this problem, an improved Kepler-type viewfinder using a split porro-prism has been proposed, in which the incident surface of the first prism has optical attributes so as to serve as a field flattener (as described in Unexamined Published Japanese Utility Model Application No. 226616/1988). However, this viewfinder still has problems of how to further enhance the image magnification and how to shorten the overall length of the finder (e.g., in the longitudinal direction).
According to the analysis conducted by the present inventors, there are several reasons for these difficulties. First, in the system described in Unexamined Published Japanese Utility Model Application No. 226616/1988, the emergent surface of the first prism and the incident surface of the second prism are both flat. Hence, the two prisms cannot be spaced apart practically. If the prisms were to be spaced apart, the size of the second prism would have to be increased sufficiently to admit all of the divergent light rays from the first prism, and thus achieving an adequate increase in the image magnification would be difficult even if the focal length of the eyepiece was increased. Another reason for the above-described difficulties is that the optical path length cannot be increased in a lateral direction, and thus shortening the longitudinal length of the viewfinder is difficult.
In view of the above-described problems of the conventional systems, an object of the present invention is to provide a Kepler-type erect image viewfinder which enhances image magnification and satisfactorily compensates for field curvature, and which also has an overall length smaller than that of the conventional systems.